This invention relates to sputtering targets useful for making magneto-optical films and, more particularly, to a unique method for preparing rare earth-transition metal targets for producing magneto-optical thin film recording materials.
Sputtering is widely accepted as the most practical method for generating magneto-optical film, making the availability of suitable target materials critical to the success of this technology. Generally, targets are prepared in one of three ways. In one process, an alloy is produced by melting two dissimilar metals in an electric arc furnace in vacuo or in an inert gas atmosphere. In a second process, an alloy is produced by incorporating rare earth metal chips into a transition metal sheet or transition metal chips into a rare earth metal sheet. A third process involves producing an alloy ingot, generally by arc melting and reducing the ingot to fine particles which are then hot pressed to form the target.
These processes and the targets produced by them have certain disadvantages. For example, the first method tends to segregate trace elements during arc melting. The alloy product can have residual cavities and casting defects and, because it is brittle, the alloy cannot be forged, heat treated, or subjected to other processes conventionally used to provide homogeneity. The size of the target is limited by the size of the electric arc furnace as is the shape of the target, necessitating subsequent working such as cutting, grinding or milling to obtain the desired configuration. The targets produced by this process have relatively high oxygen contents so that thin films produced from them are not particularly adapted to the formation of perpendicular magnetization for magneto-optical recording. Because they tend to have low tenacities, targets produced by this method crack easily, especially under thermal shock applied during the sputtering process. This characteristic is associated with rate of cooling, microstructure, and internal stress. Slow cooled castings can show large grains and phase segregation.
The second process can provide abnormal discharge between the sheet and chip materials and non-homogeniety in the alloy. Trace alloying elements on the sheet can make it difficult to obtain a film of uniform composition and, like the targets produced by the first method, the deposition rate obtained by sputtering in a magnetron, for example, is relatively slow.
The main problem with the third process is that the target material is made from a powder of an intermetallic and tends to be brittle. Oxides also tend to form on the powder surface so that the product can have a higher oxygen content than targets produced by the first two methods.